A fluorescent lamp and a high-intensity gas discharge lamp have advantages of high ruminating efficiency, long life, wide power range, etc., such that they have occupied an important position in illumination field at present. In course of illuminating the fluorescent lamp and the high-intensity gas discharge lamp, electronic beam produces light when passing through a gas medium. However, the illumination of such lamps needs to use the ballast. Because the electronic ballast has advantages of high efficiency, small volume and flexible control, it has replaced the traditional inductive ballast more and more.
How to drive bipolar transistors in half bridge topology for electronic ballast and electronic transformer and keep them in soft switching state at any operating frequency is very significant for reducing source current and power consumption. In prior art, the method for addressing this problem is to sense rising edge of voltage in the middle of half bridge and compare it with an internal signal to decide whether driving for switching bipolar transistors is too strong or too weak, thus keeping these transistors in soft switching state. Then, the current level for driving transistors is regulated so as to obtain proper driving for transistors. However, such control logic is useful only in course of normal operation, and it's not available during preheating and ignition.